This invention relates to an oxygen concentration-sensing device mainly intended for use in internal combustion engines for automotive vehicles, and more particularly to an oxygen concentration-sensing device of this kind which has a heater for heating the body of a sensor thereof.
It is well known to sense the concentration of oxygen in exhaust gases emitted from an internal combustion engine for automotive vehicles, and control the air-fuel ratio of a mixture supplied to the engine in a feedback manner responsive to the sensed oxygen concentration.
An oxygen concentration-sensing device of this kind is known e.g., from Japanese Provisional Patent Publication (Kokai) No. 61-35347, which has an oxygen concentration sensor provided with a heater which serves to promote the oxygen ion-conductivity of a solid electrolyte material forming the sensor body and hence enhance the electromotive characteristic of same.
According to the known device, a bridge circuit is used for controlling the resistance value of the heater to a predetermined value and hence the temperature of the sensor body to a desired value, in order to achieve accurate detection of the air-fuel ratio of the mixture.
However, in the known device, variation in resistance value between individual heaters used causes variation in temperature between the individual heaters and hence variation in temperature between individual sensor bodies, which results in variation in the output between individual sensors. The variation in resistance value mainly takes place during the manufacture of the heaters.
Particularly in a sensor of the proportionaloutput type which generates an output varying in proportion to the oxygen concentration, the variation in the heater resistance value exerts a great influence upon the output charateristic of the sensor, in that the sensor output is largely different between individual sensors even if they are heated to the same temperature. As a result, it is impossible to effect proper air-fuel ratio control.
Another is also known an oxygen concentrationsensing device from Japanese Provisional Patent Publication (Kokai) No. 57-20394-0, which, circuit formed by a heater for heating a sensor body, a first resistance provided on the heater side, and second and third resistances provided on the side of a temperature control circuit, wherein imbalance in potential of the bridge circuit is detected, and a voltage supplied to the heater is controlled depending upon the detected imbalance, so as to control the temperature of the sensor to a target value. According to this device, a resistance having a resistance value corresponding to the temperature vs. current characteristic of the heater, e.g., a resistance value RH.sup.o of the heater at room temperature, is selected as the first resistance so that the temperature of the heater and hence the sensor can be maintained at a target value, even if there is variation in temperature vs. current characteristic between individual heaters.
However, in the known oxygen concentrationsensing device, the resistance value of the first resistance has to be set at a value K (constant value) times as large as the resistance value of the heater at room temperature, that is, the former should be proportional to the latter. Further, the maximum allowable range of variation in the temperature vs. current characteristic between individual heaters of this kind is small. Therefore, the first resistance sometimes has to be selected from among ones with special values. To this end, it is necessary to use custom-made resistances with special resistance values, resulting in high manufacturing costs.